So-called joystick lever systems have been known as steering operation devices for controlling the steering angle of vehicles. In the steering operation device of a joystick lever system, the steering angle of a vehicle can be varied by an operator tilting a joystick lever provided to a driver seat.
The steering operation device disclosed in Japanese Laid-Open Patent Application No. 11-105723 includes a pair of hydraulic cylinders, a steering valve, and a pilot valve, for example.
The hydraulic cylinders extend between the front and rear parts of the vehicle body. The angle between the front and rear parts of the vehicle body, namely, the steering angle, is varied by elongating and contracting the hydraulic cylinders.
The steering valve adjusts the flow rate of oil supplied to the hydraulic cylinders according to the input pilot pressure.
The pilot valve has an operation input shaft and a feedback input shaft capable of rotating relative to each other. The operation input shaft is linked to a joystick lever and is rotated according to the tilt angle of the joystick lever. The feedback input shaft is linked to the front part of the vehicle body by a link member. The feedback input shaft is rotated by the transmission of a change in steering angle to the feedback input shaft by the link member. The feedback input shaft is thereby rotated according to the steering angle. The pilot valve adjusts the pilot pressure input to the steering valve according to the difference between the rotation angle of the operation input shaft and the rotation angle of the feedback input shaft. The steering operation device operates in the following manner when the joystick lever is operated.
The operation input shaft of the pilot valve is first rotated according to the tilt angle when the joystick lever is tilted in one direction. At that time, the steering angle has not yet been changed, and the rotation angle of the feedback input shaft is zero. The pilot valve is therefore opened at an aperture that corresponds to the rotation angle of the operation input shaft, and the pilot pressure is supplied to the steering valve. The steering valve supplies oil to the hydraulic cylinders at a flow rate that corresponds to the input pilot pressure. The hydraulic cylinders are thereby driven and the steering angle is changed.
The link member is moved by the change in the steering angle, and the movement of the link member is transmitted to the feedback input shaft when the steering angle is changed in agreement with the tilting operation of the joystick lever, as described above. The difference between the rotation angle of the operation input shaft and the rotation angle of the feedback input shaft is zero when the steering angle coincides with the tilt angle of the joystick lever. The pilot valve is thereby closed, and the steering valve is also closed. The change in steering angle is therefore stopped, and the steering angle is fixed in a state that matches the tilt angle of the joystick lever.
In the aforementioned steering operation device, a steering angle that corresponds to the tilt angle of the joystick lever can thus be obtained and the operability improved.